internal/strconv: add testbase tests

internal/strconv: add testbase tests

Add ability to test against inputs chosen by from the stress
tests computed by Vern Paxson's testbase program.

Checked that 'go test -testbase' passes.

Change-Id: I81057e55df6cd369b40ce623a59884e6ead0ed76
Reviewed-on: https://go-review.googlesource.com/c/go/+/719620
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>